While similar to a conventional "ROM" (Read Only Memory), an "EEPROM" (Electronically Erasable Programmable Read Only Memory) is a slightly different beast.
ROMs
By definition, a ROM is a "read only" memory. That means that it can only be
programmed/recorded once. Though the expression ROM typically refers to a computer
chip, a typical recordable CD is a ROM; it can be read many times but
can only be recorded/written once. The program that runs in a JamMan
resides on a ROM chip that is programmed/recorded/written at the factory. Nintendo
game cartridges consist of ROMs that contain the games.
One of the most important attributes of a ROM is that it saves the information stored in it even if power is not applied to the chip. RAM (Random Access Memory) chips are very similar to ROMs except that all data is lost if power is not applied the chip.
PROMs
One of the most common variations of the basic ROM is a PROM: Programmable
Read Only Memory. This just means that the chip does not have to be programmed
by the chip manufacturer. Data (a program for instance) can be recorded
into a PROM by a company that uses the chips. The PROM in JamMan is not
created by Lexicon but is programmed by Lexicon before it
is put into a JamMan.
Programming a PROM is in some ways like making an audio recording; information is transferred from one place to another. In the case of a PROM, the information is transferred from a computer (in the form of a file) into the PROM chip. The process of recording a PROM is typically refereed to as "burning a ROM" which I believe derives from the process that is going on in the chip (tiny links are being burned to save the data). The device which is used to program a PROM is usually referred to as a ROM "Burner".
EPROMs
Another common variation of the ROM is the EPROM: Erasable Programmable
Read Only Memory. This device is exactly like a PROM except that it can
be erased by exposing it to ultraviolet light for 20-40 minutes. Typically,
EPROMs have a small circle of glass on their top that allows the ultraviolet
light to reach the innards of the chip.
Though an erasable read only memory sounds like an oxymoron, the apparent contradiction is justified by the fact that the entire ROM must be erased at once. You cannot erase part of an EPROM, it's all or nothing.
EEPROMs
..which finally brings us to EEPROMs: Electronically Erasable Programmable
Read Only Memory. An EEPROM is basically the same as an EPROM except that
the ultraviolet light is not required to erase it. EEPROMs can typically
be erased by sending specific commands to the chip or by setting one of
the pins of the chip to a specific voltage.
The most important attribute of an EEPROM is that it can be erased without
ultraviolet light but still saves all of it's data if power is not applied
to the chip.